Backup your actual iptables rules.

root@debian:/# iptables-save > /etc/iptables.rules

Create a systemd service for iptables

root@debian:/# cat /etc/systemd/system/iptables.service 
[Unit]
Description=Firewall
After=network.target
 
[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/bin/sh -c "/sbin/iptables-restore < /etc/iptables.rules"
 
[Install]
WantedBy=multi-user.target

Enable the new service

root@debian:/# systemctl enable iptables.service
root@debian:/# systemctl restart iptables.service
 
root@debian:/# systemctl list-unit-files | grep iptables 
iptables.service                           enabled